This invention relates to procedures for generating lithologic logs of subterranean rock formations penetrated by a borehole from in situ measurement. More particularly, this invention relates to natural gamma ray well logging techniques.
Various methods have been used in the well-logging art to study the characteristics of subsurface formations by measuring the radioactivity of formations, both where the radioactivity is natural and where it is artificially induced. Generally, such methods applied to natural gamma ray well logging utilize three channels of spectra centered on the 1.46 MeV potassium-40, the 1.76 MeV uranium, and the 2.62 MeV thorium energies. Potassium, uranium and thorium are found in subsurface formations and potential reservoir rocks. In particular, potassium occurs in certain clay minerals and evaporites, while uranium is associated with depositional environments under reducing conditions or with movement of subsurface fluids over geologic time. Thorium, on the other hand, often occurs in certain formations containing detrital deposits. However, certain formations are notably thorium poor, such as the well-known Monterey formation of California. When present in sufficient quantities, potassium-40, uranium and thorium, or their daughter products, can be detected by gamma ray spectroscopic methods which pick up these characteristic channels of spectra and display the information in conventional logs.
Various techniques have been developed for utilizing the logs to locate source rocks for oil production. One such technique described in U.S. Pat. No. 4,017,755 to Supernaw et al. utilizes natural gamma ray logs to determine the organic carbon contained within a shale deposit. The gamma ray logs are used to derive potassium/uranium or thorium/uranium ratios which, when compared with the organic carbon values at selected depths within the borehole, are used to derive curves indicating organic carbon contained at depth within the shale deposit. In thorium-poor formations, however, this well logging technique is unsatisfactory since it depends upon ratios of all three commonly occurring radioactive nuclides.
Logging techniques may also be used to determine the amount of residual oil in an oil-bearing formation after completion of primary production. In U.S. Pat. No. 3,894,584 to Fertl a method is disclosed whereby the amount of residual oil in a formation is determined by conducting a series of three loggings. First, the formation is logged to determine its natural radioactivity. This log serves as a base or reference log. Then an aqueous solution containing water-soluble radioactive contaminants is injected into the formation through the wellbore. The formation is logged a second time with the radioactive fluids in place and a third time after the radioactive fluids have been completely displaced by water from the area surrounding the wellbore. By comparing measurements of the logs with data concerning formation porosity and radioactive concentrations employed, the amount of residual oil in the formation can be determined.
Natural gamma ray logging can also be used to examine in situ formations penetrated by a borehole to determine the optimum location for performing stimulation procedures in an uncased wellbore or for perforating a casing. Such techniques are disclosed in U.S. Pat. No. Re. 31,122 to Fertl. A natural gamma ray log of the formation surrounding the borehole is made and separated into potassium-40, uranium, and thorium energy-band signals. A differential value is derived by subtracting the signal for either potassium or thorium from the signal for urnanium at depth points along the borehole. The differential is then compared to an energy level standard to constant magnitude predetermined by the lithology of the formation. Those areas having a differential value greater than the magnitude of the standard are selected for well stimulation procedures.
One particular well-logging application, and the one to which the present invention is most directly relevant, utilizes gamma ray logging of subterranean formations to locate zones of high permeability. For formations penetrated by a cased borehole, U.S. Pat. No. 3,503,447 to Hamby discloses a process for detecting and plugging zones of high permeability, such as thief zones, to normalize injectivity along an interval into which a well is completed. Radioactive particles, such as particles of anion exchange resin carrying one-half millicurie of iodine 131 per barrel of solution, are suspended in an aqueous solution to be injected into a freshly drilled wellbore. Sugar is added to equalize the specific gravity of the resin particles and the solution so that the particles remain suspended with even distribution. The radioactive particles are large enough to plate out along the face of the wellbore as the liquid filtrate from the injected solution penetrates the formation at points of high permeabililty.
Before injection of the fluid containing radioactive particles, a log is made of background radiation occurring naturally along the borehole. Then the solution bearing radioactive particles is injected and the cake of radioactive particles plated out along the wellbore is logged. Comparison of the logs determines the location of high permeability zones, which can be plugged to prevent loss of circulation fluids during drilling or other reservoir operations. This method provides the advantage of requiring little information about the subterranean formation, such as the lateral penetration and geological composition. However, its usefulness is limited to wells already encased, perforated, and undergoing production.
The disadvantage of this method is that logging takes place after two different fluids have contacted the formation and damaged the permeability of the area surrounding the wellbore. Generally, permeability of the formation surrounding the wellbore contacted by drilling fluid is altered to some degree. When a subsequent fluid is injected to log the formation, further alteration of the reservoir permeability occurs. In addition the mechanical operations involved in installing the casing can result in damage to the permeability of the area surrounding the borehole. As a result the log made after the formation has been contacted by two fluids reflects the altered permeability surrounding the wellbore rather than the true permeability of the formation as a whole.
As illustrated by Hamby, when well logging is used to determine zones of high permeability in a formation, the log can be made inaccurate if the formation has been contacted by fluids administered before the logging operations. Inaccuracy in formation logs can also result from performing logs through a standing column of radioactive fluid in the wellbore if the concentration of radioactivity in the mud is relatively high in relation to the intensities of radiation detected by the permeability log. Certain of the weighting materials used in drilling muds, such as barite, occur naturally together with radioactive compounds, depending upon the particular formation and depth at which weighting materials are mined, as well as a number of other variables. These radioactive compounds can be adsorbed on the mud-weighting minerals or in some instances incorporated into the crystal lattice of the minerals. More rarely, radioactive minerals, such as lanthanide-containing bastnasites, are found mechanically mixed in geological formations containing substantial proportions of a mud-weighting mineral, such as barite.
Radioactive emitting nuclides from drilling muds which penetrate the formation or build up in mud cake along the wellbore have been found to distort well logging measurements used to determine a natural radioactive profile by setting up extraneous background concentrations. Barites, which are the most commonly used of all mud weighting minerals, are particularly susceptible to impurities which emit gamma radiation such as actinium (Ac.sup.228), lead (Pb.sup.212), thallium (Tl.sup.208), and, most particularly, all the daughter products of the thorium decay chain (Th.sup.232). The natural gamma ray spectral logging technique is particularly affected by radioactive barites in drilling mud because this technique relies on relatively delicate measurements of the ratios of natural gamma radiation from thorium, uranium, and potassium.
To provide a way of correcting well logging measurements to take into account the background effect of radioactive materials in the drilling mud, European Patent Application No. 83301851.8, to Smith, et al. published under Publication No. 0091294A2, discloses a test cell for measuring the radioactivity of materials in drilling mud during drilling. Extraneous background concentrations are determined by this method and measurements made by gamma ray logging are corrected for the presence of radioactivity in the mud.
Certain other techniques take advantage of the filter cake left on the wellbore when filtrate from drilling mud penetrates the wellbore under the hydrostatic pressure exerted by a column of weighted mud. U.S. Pat. No. 3,424,903 to Lawson utilizes this phenomenon to determine the permeability of subterranean formations. The drilling fluid is provided with radioactive material capable of emitting both high and low energy gamma rays in the ranges from zero to 0.8 MeV and from 1.0 to 5 MeV respectively, such as radioactive isotopes of cadmium, iron and antimony. A log is made of the high and low energy radiation emitted from filtrate from the drilling fluid bearing the radioactive material, which penetrates into the formation during drilling, and a ratio of the low to high intensities is computed. The higher attenuation of high energy gamma rays with increasing penetration into the formation is used to indicate the permeability of the formation.
This method possesses the advantage of determining permeability during drilling operations directly from the penetration of drilling mud into the formation, rather than from a subsequently injected fluid so that the permeability profile reflects more nearly an undamaged rather than a damaged or altered state. However this method also possesses several drawbacks. For one, the radioactive additive must be chosen so that it remains evenly distributed throughout the drilling fluid and does not adhere to every substance with which it comes into contact. For another, care must be taken that the radioactive fluid is not displaced so deeply into the formation that it cannot be detected with certainty. Yet another drawback of this method stems from the difficulty of logging the filtrate within the formation through a column of radioactive mud standing in the borehole and the filter cake along the face of the reservoir. Radiation from mud in the borehole can affect accuracy of the logging and mask the contours of the injection profile.
To avoid the disadvantages associated with logging through a column of mud, U.S. Pat. No. 2,810,076 to Mardock discloses a method of determining permeability by plating out radioactive particles suspended within an aqueous carrier fluid upon the face of the wellbore as the carrier fluid penetrates the formation. The walls of the drill hole are first washed or scraped to remove the mud sheath deposited during drilling operations and a log is made of the natural radioactivity of the formation. Then a neutral carrier fluid bearing radioactive particles of a size suitable for plating out on the face of the formation is injected under conditions of continuous agitation to keep the particles suspended. The carrier fluid is displaced into the formation by injecting a neutral fluid maintained under pressure to prevent backflow from the formation, and a second log of the borehole bearing the cake of deposited particles is made. Comparison of radioactive profiles of the first and second logs will reveal the zones of high permeability. However, if fluid from the reservoir backflows into the borehole, the cake of particles will be damaged or displaced and results of the second logging will be highly inaccurate.
One of the main problems of the prior art logging methods is that none provides a reliable way to determine the permeability of a formation before it is damaged. For example, the process of Hamby described above relates to the logging of formations penetrated by a cased borehole; obviously, the operations involved in installing the casing damage the formation so that the results obtained from logging do not accurately reflect the permeabililty characteristics of the formation. Likewise, the logging fluids used by Hamby, and also by Mardock in the patent discussed above, will alter the formation so that the permeability of the area immediately surrounding the wellbore, the area read by the logging tool, no longer relfects the true permeability characteristics of the formation.